Hey everyone,

I have a question about upgrading to 3.70 rear gear (I have the stock 3.27 right now) and I was just wondering if it is worth it?

In a stock 700R4 how high will the RPMs be on the freeway?

and how bad does it throw off the speedometer?

Thanks guys

 

Define "worth" "it".

Your problem will not be highway RPMs. Instead, TPI being the hard and inevitable RPM limiter that it is, the engine will reach TPIs "brick wall" RPM in a flash, and then the car will sit there without further acceleration and wait for the transmission to catch up. Similarly, you will be driving down a 2-lane road behind an 18-wheeler, pull out to pass, it'll downshift, and the car will slow down from the drag of trying to pull the engine to a higher RPM. You'll have one of those wonders that'll absilutely burn the tires off sitting still, but ... is all played out by about 10 mph.

Your current gear is about optimum for an otherwise unmodified L98.

Going from 3.27 to 3.70 will "throw off the speedometer", unless you get the correct speedo gears as well, by a factor of about 3.70 / 3.27... which is to say, it'll read a little more than 13% above your actual road speed at all times.

 

Thanks for your help

and yeah I was afraid of that TPI problem being dead after 4,000 RPMs, I was just wondering because those rare G92 firebird had 3.70 from the factory

anyway thanks again for your help

 

Stock L98s should pull 4800 before hitting the wall. If you're feeling it around 4000, then you have other problems to correct. TPI is not inherently limiting. There's a long-tube TPI 383 peaking around 6200, check my old Best Builds thread that used to be a sticky in the Engine Swap sub-forum if you want details.

 

AFAIK the only factory cars with those gears were L69 Firebirds - not Camaros, the 9-bolt didn't start getting used in them until later - in 85 and (if there were any? can't swear to it) 86. Never used with TPI. Highest gear ever used AFAIK with TPI was 3.45, and that was only in Camaro and Firebird with 305 / 5-spd and G92.

 

The 3.70 gears were used on 85-86 trans ams with 5.0 liter HO engine and 5 speed T5.
I found two of those diamonds in the rough at the junkyard and they were picked immediately!!! The HO engines were higher revving engines that made power at a higher rpm range than tpi. I had 3.73's and my launch was awesome!!! Highway speed at 70 mph i would spin 2400 rpm's on 26 inch wheels. I have a T5 and yours is auto so just stick to your 3.27's cause the you wont gain much ET. I know cause i run my car at the track and i get out of the whole faster but my engine runs out of steam after third gear.